Ohm's law is a mathematical expression that describes the relation between potential difference (voltage), electric current, and resistance. In order for a material to obey Ohm's law the resistance must remain constant - meaning that the current and potential difference must be proportional to each other. While Ohm's law is actually only an approximation, it works very well over an incredibly wide range of voltages. With that being said, it is possible to change the resistance of a circuit, but the only way to do so is to physically install/remove additional resistors before using or analyzing the circuit:
The University of Colorado has graciously allowed us to use the following Phet simulation. This simulation explores Ohm's Law. Note: This simulation allows the adjustment of resistance; however, notice when you adjust the voltage only the current changes (proportionally). This implies that resistance remains constant. What this simulation allows you to do is change the actual physical properties of the resistor in order to see how different resistors affect circuits. In reality it isn't possible to change the resistance without changing the physical properties of the circuit.
For further information please see the related pages below: